The present invention relates to a method of blending an additive into a base polymer. In particular, it relates to producing a homogeneous polymer blend with an additive, especially a salt additive useful for the manufacture of polymer films.
Melt blending polymers and functional additives to produce blended compounds is a common method to deliver those additives during the manufacture of plastic articles. The compound may be a concentrated combination (or masterbatch) of the additive and a carrier material whereby the final amount of the additive in the article is achieved by dilution during fabrication of the article. Additionally, the compound may be a fully formulated blend of additives and polymers that directly reflects the composition of the finished article. The quality and functionality of the article is often impacted by the uniformity of the dispersion of the additives in the compound. If the homogeneity of the dispersion is poor, it can be expected that the intended functionality of the additive will vary. Additionally, it is possible that undesirable side effects like poor aesthetics may result from poor dispersion of additives.
A pellet is a convenient form for providing compounded materials. The traditional way to form a compounded pellet is to combine the individual component materials in a high intensity mixing device. The polymeric portion of the combination is melted to form a viscous liquid or “melt”. Various additives may be combined with the polymer before, during, or after melting. Those additives may be solids that may or may not undergo a solid to liquid (melting) phase change, solids that may or may not go through a solid to gas (sublimation) phase change, liquids that may or may not undergo a liquid to gas (vaporization) phase change, or gases. The high intensity mixing device is used to attempt to uniformly disperse the additive within the host polymer or carrier material. After additive dispersion is complete, the melt is discharged through a shaping device or “die” that is used to prepare the final shape of the compound. To form the convenient pellet shape, the die typically is outfitted with circular orifices through which the molten compound flows. The circular orifices form continuous cylinders of the compound that are subsequently cut to form pellets.
One type of additive is a colorant which may be a dye or solid pigment that may be compounded into a base polymer to form a masterbatch of colorant in a carrier polymer. It is known to produce such colorant masterbatches by addition of the colorant directly with the polymer, with or without premixing, into a Banbury mixer or a single or twin screw extruder hopper and thereby into an extruder which mixes the additive and carrier polymer together to form the masterbatch. Another known method is to disperse pigment into a liquid carrier such as mineral oil and admit the dispersion into the extruder hopper. Also, solid additives such as minerals e.g. calcium carbonate, silica and the like, and pigments, etc. as well as nonaqueous liquids such as tackifiers, antioxidants, slip agents and antifog agents may be introduced directly into a polymer melt to form an article or masterbatch pellets. It has also been suggested to introduce water alone into a polymer melt to act as a foaming agent.
The quality and functionality of dispersed additives depends upon certain characteristics of the additive and polymeric carrier as well as the mixing device. In the case of dispersing solid additive particles in a polymeric carrier, the distribution of sizes of the particles has a primary impact on the homogeneity of the resultant compound and fabricated articles that include that compound. The degree of homogeneity can, in turn, affect the functionality of the finished article. Additionally, it is desirable that the configuration of the mixing device and its conditions of use be appropriately selected such that the total surface area of the particles contacted by the carrier polymer is maximized. It is generally regarded that the presence of poorly wetted particle agglomerations is undesirable. Therefore, there is a need for a method of introducing additives into a polymer to provide a homogeneous blend.